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Arc lamp
Arc lamp
Arc lamp
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The 15 kW xenon short-arc lamp used in the IMAX projection system.
A mercury arc lamp from a fluorescence microscope.
A krypton long arc lamp (top) is shown above a xenon flashtube. The two lamps, used for laser pumping, are very different in the shape of the electrodes, in particular, the cathode (on the left).

An arc lamp or arc light is a lamp that produces light by an electric arc (also called a voltaic arc).

The carbon arc light, which consists of an arc between carbon electrodes in air, invented by Humphry Davy in the first decade of the 1800s, was the first practical electric light.[1][2] It was widely used starting in the 1870s for street and large building lighting until it was superseded by the incandescent light in the early 20th century.[1] It continued in use in more specialized applications where a high intensity point light source was needed, such as searchlights and movie projectors until after World War II. The carbon arc lamp is now obsolete for most of these purposes, but it is still used as a source of high intensity ultraviolet light.

The term is now used for gas discharge lamps, which produce light by an arc between metal electrodes through a gas in a glass bulb. The common fluorescent lamp is a low-pressure mercury arc lamp.[3] The xenon arc lamp, which produces a high intensity white light, is now used in many of the applications which formerly used the carbon arc, such as movie projectors and searchlights.

Operation

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An arc is the discharge that occurs when a gas is ionized. A high voltage is pulsed across the lamp to "ignite" or "strike" the arc, after which the discharge can be maintained at a lower voltage. The "strike" requires an electrical circuit with an igniter and a ballast. The ballast is wired in series with the lamp and performs two functions.

When the power is first switched on, the igniter/starter (which is wired in parallel across the lamp) sets up a small current through the ballast and starter. This creates a small magnetic field within the ballast windings. A moment later the starter interrupts the current flow from the ballast, which has a high inductance and therefore tries to maintain the current flow (the ballast opposes any change in current through it); it cannot, as there is no longer a 'circuit'. As a result, a high voltage appears across the ballast momentarily, to which the lamp is connected; therefore the lamp receives this high voltage across it which 'strikes' the arc within the tube/lamp. The circuit will repeat this action until the lamp is ionized enough to sustain the arc.

When the lamp sustains the arc, the ballast performs its second function, to limit the current to that needed to operate the lamp. The lamp, ballast, and igniter are rating-matched to each other; these parts must be replaced with the same rating as the failed component or the lamp will not work.

The colour of the light emitted by the lamp changes as its electrical characteristics change with temperature and time. Lightning is a similar principle where the atmosphere is ionized by the high potential difference (voltage) between earth and storm clouds.

A krypton arc lamp during operation.

The temperature of the arc in an arc lamp can reach several thousand degrees Celsius. The outer glass envelope can reach 500 degrees Celsius, therefore before servicing one must ensure the bulb has cooled sufficiently to handle. Often, if these types of lamps are turned off or lose their power supply, one cannot restrike the lamp again for several minutes (called cold restrike lamps). However, some lamps (mainly fluorescent tubes/energy saving lamps) can be restruck as soon as they are turned off (called hot restrike lamps).

The Vortek water-wall plasma arc lamp, invented in 1975 by David Camm and Roy Nodwell at the University of British Columbia, Vancouver, Canada, made the Guinness Book of World Records in 1986 and 1993 as the most powerful continuously burning light source at over 300 kW or 1.2 million candle power.[4]

Carbon arc lamp

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A carbon arc lamp, cover removed, on the point of ignition. This model requires manual adjustment of the electrodes
An electric arc, demonstrating the “arch” effect.
Early experimental carbon arc light powered by liquid batteries, similar to Davy's
Medical carbon arc lamp used to treat skin conditions, 1909
Self-regulating arc lamp proposed by William Edwards Staite and William Petrie in 1847

In a carbon arc lamp, the electrodes are carbon rods in free air. To ignite the lamp, the rods are touched together, thus allowing a relatively low voltage to strike the arc.[1] The rods are then slowly drawn apart, and electric current heats and maintains an arc across the gap. The tips of the carbon rods are heated and the carbon vaporizes.[1] The rods are slowly burnt away in use, and the distance between them needs to be regularly adjusted in order to maintain the arc.[1]

Many ingenious mechanisms were invented to control the distance automatically, mostly based on solenoids. In one of the simplest mechanically-regulated forms (which was soon superseded by more smoothly acting devices) the electrodes are mounted vertically. The current supplying the arc is passed in series through a solenoid attached to the top electrode. If the points of the electrodes are touching (as in start up) the resistance falls, the current increases and the increased pull from the solenoid draws the points apart. If the arc starts to fail the current drops and the points close again.

The Yablochkov candle is a simple arc lamp without a regulator, but it has the drawback that the arc cannot be restarted (single use) and a limited lifetime of only a few hours.

Spectrum

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The spectrum emitted by a carbon-arc lamp is the closest to that of sunlight of any lamp. One of the first electric lights, their harsh, intense output usually limited their use to lighting large areas. Although invisible wavelengths were unknown at the time of their invention, unenclosed lamps were soon discovered to produce large amounts of infrared and harmful ultraviolet radiation not found in sunlight. If the arc was encased in a glass globe, it was found that many of these invisible rays could be blocked. However, carbon-arcs were soon displaced by safer, more efficient, versatile, and easier to maintain incandescent and gas-discharge lamps. Carbon-arc lamps are still used where a close approximation of sunlight is needed, for testing materials, paints, and coatings for wear, fading, or deterioration, or, for example, spacecraft materials that are to be exposed to sunlight at orbits closer than Earth's.[5]

The arc consists of pure carbon-vapor heated to a plasma state. However, the arc contributes very little of the light output, and is considered non-luminous, as most of its emission occurs in spectral lines in the violet and UV portions of the spectrum. Most of the carbon spectra occurs in a very broad line centered at 389 nm (UV-A, just outside the visual spectrum), and a very narrow line at 250 nm (UV-B), plus some other less-powerful lines in the UV-C.

Most of the visible and IR radiation is produced from incandescence created at the positive electrode, or anode. Unlike the tungsten anodes found in other arc lamps, which remain relatively cool, carbon produces much higher resistance and the electrons are forced to enter the anode at the hottest point, generating tremendous amounts of heat that vaporizes the carbon and creates a pit in the anode's surface. This pit is heated from 6000 to 6500 degrees Fahrenheit (3300 to 3600 degrees Celsius, just below its melting point), causing it to glow very brightly with incandescence. Due to this, the electrodes were often placed at right angles from each other with the anode facing outward to keep from blocking its light output. Since carbon has the highest melting point of any element, it is the only lamp whose blackbody radiation is capable of nearly matching the Sun's temperature of 10,000 degrees Fahrenheit (5500 degrees Celsius), especially when filters are used to remove most of the IR and UV light.[6]

History

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The concept of carbon-arc lighting was first demonstrated by Humphry Davy in the early 19th century, but sources disagree about the year he first demonstrated it; 1802, 1805, 1807 and 1809 are all mentioned. Davy used charcoal sticks and a two-thousand-cell battery to create an arc across a 4-inch (100 mm) gap. He mounted his electrodes horizontally and noted that, because of the strong convection flow of air, the arc formed the shape of an arch. He coined the term "arch lamp", which was contracted to "arc lamp" when the devices came into common usage.[7]

In the late nineteenth century, electric arc lighting was in wide use for public lighting. The tendency of electric arcs to flicker and hiss was a major problem. In 1895, Hertha Ayrton wrote a series of articles for The Electrician, explaining that these phenomena were the result of oxygen coming into contact with the carbon rods used to create the arc.[8][9] In 1899, she was the first woman ever to read her own paper before the Institution of Electrical Engineers (IEE). Her paper was "The Hissing of the Electric Arc".[10]

The arc lamp provided one of the first commercial uses for electricity, a phenomenon previously confined to experiment, the telegraph, and entertainment.[11]

Carbon-arc lighting in the U.S.

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In the United States, there were attempts to produce arc lamps commercially after 1850, but the lack of a constant electricity supply thwarted efforts. Thus electrical engineers began focusing on the problem of improving Faraday's dynamo. The concept was improved upon by a number of people including William Edwards Staite [de] and Charles F. Brush. It was not until the 1870s that lamps such as the Yablochkov candle were more commonly seen. In 1877, the Franklin Institute conducted a comparative test of dynamo systems. The one developed by Brush performed best, and Brush immediately applied his improved dynamo to arc-lighting, an early application being Public Square in Cleveland, Ohio, on April 29, 1879.[12] Despite this, Wabash, Indiana claims to be the first city ever to be lit with "Brush Lights". Four of these lights became active there on March 31, 1880.[13] Wabash was a small enough city to be lit entirely by 4 lights, whereas the installation at Cleveland's Public Square only lit a portion of that larger city.[14] In 1880, Brush established the Brush Electric Company.

The harsh and brilliant light was found most suitable for public areas, such as Cleveland's Public Square, being around 200 times more powerful than contemporary filament lamps.

The usage of Brush electric arc lights spread quickly. Scientific American reported in 1881 that the system was being used in:[15] 800 lights in rolling mills, steel works, shops, 1,240 lights in woolen, cotton, linen, silk, and other factories, 425 lights in large stores, hotels, churches, 250 lights in parks, docks, and summer resorts, 275 lights in railroad depots and shops, 130 lights in mines, smelting works, 380 lights in factories and establishments of various kinds, 1,500 lights in lighting stations, for city lighting, 1,200 lights in England and other foreign countries. A total of over 6,000 lights were actually sold.

There were three major advances in the 1880s: František Křižík invented in 1880 a mechanism to allow the automatic adjustment of the electrodes. The arcs were enclosed in a small tube to slow the carbon consumption (increasing the life span to around 100 hours). Flame arc lamps were introduced where the carbon rods had metal salts (usually magnesium, strontium, barium, or calcium fluorides) added to increase light output and produce different colours.

In the U.S., patent protection of arc-lighting systems and improved dynamos proved difficult and as a result the arc-lighting industry became highly competitive. Brush's principal competition was from the team of Elihu Thomson and Edwin J. Houston. These two had formed the American Electric Corporation in 1880, but it was soon bought up by Charles A. Coffin, moved to Lynn, Massachusetts, and renamed the Thomson-Houston Electric Company. Thomson remained, though, the principal inventive genius behind the company patenting improvements to the lighting system. Under the leadership of Thomson-Houston's patent attorney, Frederick P. Fish, the company protected its new patent rights. Coffin's management also led the company towards an aggressive policy of buy-outs and mergers with competitors. Both strategies reduced competition in the electrical lighting manufacturing industry. By 1890, the Thomson-Houston company was the dominant electrical manufacturing company in the U.S.[16]

Around the turn of the century arc-lighting systems were in decline, but Thomson-Houston controlled key patents to urban lighting systems. This control slowed the expansion of incandescent lighting systems being developed by Thomas Edison's Edison General Electric Company. Conversely, Edison's control of direct current distribution and generating machinery patents blocked further expansion of Thomson-Houston. The roadblock to expansion was removed when the two companies merged in 1892 to form the General Electric Company.[16]

Arc lamps were used in some early motion-picture studios to illuminate interior shots. One problem was that they produce such a high level of ultra-violet light that many actors needed to wear sunglasses when off camera to relieve sore eyes resulting from the ultra-violet light. The problem was solved by adding a sheet of ordinary window glass in front of the lamp, blocking the ultra-violet. By the dawn of the "talkies", arc lamps had been replaced in film studios with other types of lights.[17] In 1915, Elmer Ambrose Sperry began manufacturing his invention of a high-intensity carbon arc searchlight. These were used aboard warships of all navies during the 20th century for signaling and illuminating enemies.[18] In the 1920s, carbon arc lamps were sold as family health products, a substitute for natural sunlight.[19]

Arc lamps were superseded by filament lamps in most roles, remaining in only certain niche applications such as cinema projection, spotlights, and searchlights. In the 1950s and 1960s the high-power D.C. for the carbon-arc lamp of an outdoor drive-in projector would typically be supplied by a motor–generator combo (AC motor powering a DC generator). Even in these applications conventional carbon-arc lamps were mostly pushed into obsolescence by xenon arc lamps, but were still being manufactured as spotlights at least as late as 1982[20] and are still manufactured for at least one purpose – simulating sunlight in "accelerated aging" machines intended to estimate how fast a material is likely to be degraded by environmental exposure.[21][22]

Carbon arc lighting left its imprint on other film projection practices. The practice of shipping and projecting motion pictures on 2,000-foot reels, and employing "changeovers" between two projectors, was due to the carbon rods used in projector lamphouses having a lifespan of roughly 22 minutes (which corresponds to the amount of film in said reels when projected at 24 frames/second). The projectionist would watch the rod burn down by eye (though a peephole like a welder's glass) and replace the carbon rod when changing film reels. The two-projector changeover setup largely disappeared in the 1970s with the advent of xenon projector lamps, being replaced with single-projector platter systems, though films would continue to be shipped to cinemas on 2,000-foot reels.

See also

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References

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Bibliography

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Arc lamp

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from Grokipedia
An arc lamp, also known as an arc light, is an electric lamp that generates intense illumination by creating and sustaining an electric arc—a luminous plasma discharge—between two electrodes, most commonly carbon rods, when a high-voltage current is applied across them. The arc vaporizes the electrode tips at temperatures exceeding 3,500°C (6,300°F), producing a brilliant white light through the incandescence of the heated carbon and the excitation of surrounding ionized gases. This design marked one of the earliest forms of practical electric lighting, offering far greater brightness than contemporary gas or oil lamps, though it required constant maintenance to regulate the electrode gap as the rods consumed during operation. The concept of the arc lamp originated in the early 19th century, with British chemist Sir Humphry Davy demonstrating the first electric arc in 1801 using a battery to spark between charcoal electrodes, though practical implementation awaited advancements in power generation. Significant progress occurred in the 1840s with the invention of reliable electric generators, enabling the first lighthouse application at South Foreland in England in 1858. By the 1870s, innovations like the Jablochkoff candle—developed by Russian engineer Pavel Yablochkov in 1876, featuring parallel carbon rods separated by plaster of Paris for self-regulating burn—facilitated widespread adoption, including the illumination of the Avenue de l'Opéra with 64 units in 1878; by 1881, Paris had over 4,000 electric arc lamps in use. American inventor Charles F. Brush further refined the technology in 1879 with his dynamo-powered system, which illuminated public squares and factories, while later contributions from Elihu Thomson and others improved efficiency and longevity. Arc lamps operate primarily in direct current (DC) mode for stability, though alternating current (AC) variants exist, and come in types such as carbon arc lamps for general high-intensity needs and gas-discharge variants like xenon or mercury short-arc lamps, where an inert gas envelope enhances light output and color spectrum. These lamps excel in luminosity, capable of producing thousands of candlepower to illuminate large areas, but suffer from drawbacks including short electrode life (typically 75–600 hours), emission of ultraviolet radiation and carbon monoxide, and the need for mechanical regulators to maintain the arc. Historically, arc lamps revolutionized urban and industrial lighting in the late 19th century, powering streetlights, theaters, and searchlights—such as those used in naval applications by 1915—before being largely supplanted by the incandescent bulb in the early 20th century due to its simplicity and indoor suitability. Today, specialized arc lamps persist in applications requiring extreme brightness, including cinema projectors, solar simulators, and medical phototherapy, underscoring their enduring legacy in lighting technology.

Operation

Basic Principle

An electric arc in an arc lamp is a plasma discharge between two electrodes separated by a gas, typically air or an inert gas. The arc forms when a high voltage, usually 30–100 V, is applied across the electrodes, ionizing gas molecules and creating free electrons and ions that establish a conductive plasma channel. This channel supports a sustained discharge with temperatures reaching up to 6000 K, enabling the high-density current flow characteristic of arc operation. Light production in arc lamps arises from two primary mechanisms: thermal radiation, known as incandescence, emitted by the intensely heated electrodes, and spectral emission from excited atoms and ions within the plasma. The electrodes can reach temperatures sufficient for blackbody-like radiation across a broad spectrum, while collisions in the plasma excite gas atoms, leading to the release of photons at specific wavelengths upon de-excitation. Electrically, arc lamps require high currents of 10–100 A to maintain the discharge, with the power input determined by the equation P=V×IP = V \times I, where VV is the sustaining voltage across the arc (often 20–50 V after initiation) and II is the current. Ballast resistors or regulators are essential to limit current fluctuations, stabilize the arc length, and prevent excessive electrode consumption or instability. Early arc lamps exhibit low luminous efficacy, typically 2–7 lumens per watt, as much of the electrical energy converts to heat and ultraviolet radiation rather than visible light, limiting their overall efficiency compared to later lighting technologies.

Key Components

The core of an arc lamp consists of two electrodes, typically rods made of conductive material, positioned with a spacing of 1 to 15 mm to sustain the arc. These electrodes consume gradually during operation, necessitating periodic manual adjustment in single-unit lamps or automated feeding in multi-lamp series setups to maintain optimal gap distance. The power supply for arc lamps generally employs direct current (DC) generators or converters, with alternating current (AC) options requiring specialized ballasts for stability. Rheostats or current regulators are integral for controlling the electrical input, allowing initial high striking voltages of around 50-80 V to initiate the arc, followed by running parameters of 50-60 V and 10-20 A for typical operation. Traditional arc lamps operate in an open-air enclosure to allow heat dissipation, often incorporating basic reflectors to direct light output efficiently. Ventilation systems, such as exhaust vents or fans, are essential to remove fumes generated from electrode vaporization, preventing accumulation of harmful gases like carbon monoxide and nitrogen oxides. Auxiliary mechanisms include mechanical feed systems, such as clockwork or solenoid-driven advances, which automatically push electrodes closer as they erode in series configurations for continuous operation without interruption. In simpler single-lamp units, manual trimming of electrodes is required periodically to reset the gap. Safety features encompass insulated wiring and proper grounding to mitigate risks from high voltages and currents, alongside protective enclosures that shield operators from arc flashes and heat. Additional precautions, like barriers around open arcs, help prevent electrical shocks and exposure to intense light.

History

Early Experiments and Invention

The concept of the electric arc as a light source was first demonstrated publicly by British chemist Humphry Davy in 1801 at the Royal Institution in London. Using a massive battery consisting of over 2,000 voltaic cells, Davy passed a current between two carbon electrodes held close together, creating a sustained luminous arc that produced intense white light visible from afar. This exhibition highlighted the arc's potential for illumination but was limited to short durations due to the battery's rapid depletion after just minutes of operation. Throughout the early 19th century, arc lighting remained confined to scientific lectures and experimental setups in Europe, where researchers refined the technology for demonstration purposes. Early electrodes made from simple charcoal rods were prone to uneven burning, prompting gradual improvements toward retort carbon and, by the 1840s, more durable graphite composites that extended arc stability and reduced flickering. These advancements allowed for brighter, more reliable light in small-scale applications, such as laboratory displays, though power sources still depended on cumbersome batteries. Practical deployment began in lighthouse illumination during the mid-19th century, marking the arc's transition from novelty to utility. In 1858, the South Foreland Lighthouse in England became the first to employ an electric arc lamp, equipped with a mechanical regulator invented by French physicist Léon Foucault to automatically maintain the optimal electrode gap for continuous burning. This system, powered by a magneto-electric generator, provided a steady beam far superior to oil lamps in intensity and range. In France, similar adoption followed soon after, with arc lamps installed at the Cape La Hève Lighthouse in 1863 using a design by Victor Serrin featuring clockwork and solenoid adjustments. Inventors like French engineer Hippolyte Fontaine played a crucial role in enabling sustained arc operation by innovating regulators and integrating dynamos for more efficient power delivery, as detailed in his 1877 treatise on electric lighting systems. These efforts addressed variability in arc length but could not fully overcome inherent drawbacks, including the high voltage demands that strained early generators, the audible hissing and sputtering noise from ionized air, and the constant need for manual electrode replacement every few hours due to vaporization. Such maintenance-intensive and power-hungry characteristics confined arc lamps to specialized uses until dynamo improvements in the 1870s.

Commercial Development

The commercial development of arc lamps accelerated in the 1870s, driven by innovations that addressed reliability and scalability issues, transforming the technology from laboratory curiosity to practical urban lighting solution. A pivotal advancement was the 1876 invention of the Jablochkoff candle by Russian engineer Pavel Yablochkov, which featured two parallel carbon rods separated by a gypsum insulator, eliminating the need for mechanical electrode feeding mechanisms that plagued earlier designs. This simplified structure allowed for more consistent operation and easier replacement, making it suitable for public demonstrations. At the Paris Exposition Universelle of 1878, Yablochkov installed 64 such candles along the Avenue de l'Opéra, Place du Théâtre Français, and Place de l'Opéra, illuminating key areas and showcasing arc lighting's potential for large-scale use; within two years, over 2,500 units were deployed across Europe for street and dock lighting in cities like Paris and London. Parallel efforts in the United States advanced power generation and control systems essential for multi-lamp installations. In 1877, American inventor Charles F. Brush developed a dynamo-powered series arc lamp system, incorporating automatic electromagnetic regulators—such as solenoids and clutches—to maintain optimal electrode spacing as carbons burned down, ensuring steady illumination without constant manual adjustment. This design supported up to several dozen lamps per circuit through innovative cut-out mechanisms that isolated faulty units, enabling efficient distribution of power from a single generator and paving the way for municipal-scale deployments. Brush's system emphasized durability with copper-coated, still-coke carbon electrodes that extended operational reliability, marking a shift toward economically viable electric lighting networks. European municipalities rapidly adopted these technologies for street lighting in the late 1870s and early 1880s, establishing arc lamps as a superior alternative to gas illumination. In 1878, Jablochkov candles lit London's Thames Embankment, providing the city's first electric street lights and demonstrating brighter, more uniform coverage over gas lamps. Paris followed with permanent installations on major boulevards that same year, while Berlin introduced 36 arc lamps on Leipziger Straße in 1882, initiating widespread urban electrification in Germany. In England, Godalming experimentally deployed three Siemens arc lamps (each 300 candle-power) on 22-foot poles in September 1881, powered by a water-driven dynamo, serving as one of the earliest temporary municipal systems before permanent adoption elsewhere. These installations highlighted arc lamps' advantages in safety and intensity, though maintenance challenges like frequent electrode replacement persisted. Technological refinements in the late 1870s focused on power delivery and light quality to enhance practicality. Early arc systems predominantly used direct current (DC) for stable electrode consumption and quieter operation, but debates emerged over alternating current (AC) viability, as AC enabled longer-distance transmission with transformers—foreshadowing the "war of the currents" in the 1880s—though it required modifications to regulators for consistent arc stability. Around 1880, flame arc lamps were introduced, incorporating metal salts (such as strontium or rare-earth fluorides) into carbon electrodes to produce a downward-directed, yellowish flame with improved color rendering compared to the harsh white-blue of standard arcs, better suiting indoor and street applications by approximating natural light tones. These innovations reduced flickering and extended usability, though AC adoption remained limited until dynamo improvements in the 1880s. Economic considerations played a crucial role in arc lamps' market penetration, balancing high upfront investments against long-term savings over gas lighting. Initial setups cost around $900 for small systems with five arcs in the late 1870s, escalating to $300–$1,000 per lamp including generators and wiring for urban installations, deterring widespread adoption initially. However, electrode pairs typically lasted 100 hours in early designs—far exceeding gas mantles' frequent trimming—yielding annual operating costs of about $336 per arc by 1881, or roughly 33% more than gas but delivering 10 times the illumination. Cities like Wabash, Indiana, reported $800 annual savings per system by 1880 due to reduced labor and fuel needs, offsetting capital expenses and driving commercial viability in high-traffic areas.

Adoption in the United States

The adoption of arc lamps in the United States began with pioneering installations in the late 1870s, marking a shift from gas lighting to electric illumination in urban areas. In 1879, inventor Charles F. Brush demonstrated the first permanent public electric street lighting system in Cleveland, Ohio, installing 12 arc lamps around Public Square powered by a dynamo estimated at around 12 horsepower, which illuminated an area of several acres and drew thousands of spectators. This system represented a commercial breakthrough, as Brush's self-regulating design minimized the need for constant manual adjustment, enabling reliable operation for street lighting. Rapid expansion followed, driven by municipal interest in brighter, more efficient lighting for growing cities. In December 1880, New York City activated its first arc street lamps along Broadway, initially with a small number of Brush units spaced at intervals to light key thoroughfares. San Francisco adopted arc systems by 1881, incorporating them into early electric infrastructure to support its booming population and port activities. Chicago followed in 1882, deploying arc lamps for public streets and bridges, which enhanced nighttime visibility and safety. By 1886, arc lighting had proliferated to dozens of U.S. cities, with systems in places like Philadelphia, Boston, and smaller towns such as Wabash, Indiana, where four lamps on a central tower fully illuminated the community. Earlier contributions laid groundwork for these developments, including experiments by Moses G. Farmer, who in 1859 lighted his home in Salem, Massachusetts, every evening with subdivided electric lights using platinum elements in open air, powered by batteries—an early effort toward practical enclosed electric illumination. Farmer later advanced arc technology, including applications for challenging environments like underwater lighting for bridges, though these remained experimental. Adoption faced regulatory hurdles and economic challenges, including opposition from gas lighting lobbies that influenced local policies to protect their interests, alongside high maintenance demands such as daily electrode trimming to sustain the arc. Despite these, arc systems peaked in the U.S. around 1890 with over 130,000 lamps in operation, powering street networks in more than 70 cities and transforming urban nighttime economies. The decline began in the late 1880s with the commercialization of incandescent bulbs, which offered safer, lower-maintenance alternatives suitable for homes and finer street spacing without the intense glare or ozone production of arcs. By the 1920s, most major U.S. arc street lighting systems had been phased out in favor of incandescents and emerging technologies, though remnants persisted in specialized uses.

Types

Carbon Arc Lamp

The carbon arc lamp utilizes two tapered carbon electrodes, typically made from graphite-impregnated material, positioned end-to-end with a small gap between them to sustain the electric arc. These electrodes are connected in series or parallel configurations depending on the power supply setup for multiple lamps, and they require feed mechanisms—either manual adjustment by an operator or automatic systems using clockwork, electromagnets, or differential gearing—to gradually advance the rods as they consume during operation. The design operates in open air without enclosure for the arc, allowing atmospheric interaction that contributes to its characteristic light production. In operation, the lamp requires a direct current supply of 40-60 volts across the arc, with currents typically ranging from 5-30 amperes for smaller units and up to 90-125 amperes for high-intensity versions, often incorporating current limiters or regulators to maintain stability and prevent overload. The electrodes are initially touched to strike the arc, then separated to about 1-3 mm, where the high temperature—reaching approximately 3600°C—vaporizes carbon, creating a plasma that emits light through incandescence and excitation. Electrode consumption occurs at a rate of roughly 0.5-1 mm per minute for the positive electrode under standard conditions, necessitating continuous feeding to sustain the arc length and output. This process generates a hissing noise from gas ionization and produces soot as a byproduct of carbon oxidation, requiring ventilation in practical installations. The spectrum of the carbon arc lamp is continuous and broadband, extending from ultraviolet to infrared wavelengths, with peak emission in the blue-green region around 388 nm due to excitation of carbon vapor and atmospheric nitrogen, producing prominent CN molecular bands that impart a cyanotic tint to the light. Its luminous efficacy ranges from 2-4 lm/W, enabling high-intensity output up to 10,000 lumens in typical configurations, making it suitable for demanding illumination needs despite the inefficiencies. Unique advantages of the carbon arc lamp include its exceptional brightness and rapid startup, providing immediate high-lumen illumination without warmup, which outperformed contemporary alternatives like gas lamps in large-scale applications. However, the hissing sound and soot deposition posed operational challenges, contributing to its eventual decline. Variants include plain carbon electrodes for standard use and cored electrodes impregnated with chemicals such as metals (e.g., nickel or cobalt) to extend life, reduce consumption, or correct the cyanotic spectrum toward whiter light.

Jablochkoff Candle

The Jablochkoff candle, invented by Russian engineer Pavel Yablochkov in 1876, employed an innovative parallel-electrode configuration to simplify arc lamp operation. It consisted of two vertical carbon rods placed side by side and embedded in a block of gypsum, also known as plaster of Paris, serving as an insulator between them. The assembly was ignited at the top by striking an arc, after which the carbons burned downward progressively, consuming both the electrodes and the surrounding gypsum without requiring any mechanical feeding or adjustment mechanisms. This design allowed each candle to operate for approximately 1.5 to 2 hours before full consumption. In operation, an electric current—preferably alternating current to promote even electrode consumption—was applied across the rods, initiating and sustaining the arc at the exposed top end. As the arc progressed, the intense heat melted the gypsum insulator, gradually exposing fresh sections of the carbon rods and thereby self-regulating the arc length to maintain a relatively consistent gap. The candle typically functioned at voltages around 50 V and currents of about 10 A, generating a brilliant white light ideal for illuminating large outdoor spaces. Unlike earlier serial electrode designs, this parallel setup eliminated the need for complex regulators, making it suitable for both direct and alternating current sources. The primary advantages of the Jablochkoff candle lay in its mechanical simplicity and reliability, as it contained no moving parts and could be easily installed in series circuits for applications like temporary street lighting. This allowed multiple units—up to dozens—to be powered from a single generator over extended distances, facilitating economical deployment in urban settings without individual adjustments. However, its limitations included the disposable nature of the unit, which had to be entirely replaced after burnout, and a tendency for uneven light output toward the end of its life as the arc lengthened and stability decreased. Additionally, operation produced side effects such as carbon monoxide emissions, ultraviolet light, audible buzzing, and radio interference. Historically, the Jablochkoff candle achieved significant impact through its demonstration at the 1878 Exposition Universelle in Paris, where 64 units illuminated prominent sites including the Avenue de l'Opéra, Place du Théâtre Français, and Place de l'Opéra, marking one of the first large-scale public displays of electric arc lighting. This success popularized the concept of series-connected lighting systems and spurred adoption across Europe for docks, streets, and expositions, with over 2,500 units in use within two years. Despite its influence, the candle's short lifespan and need for daily replacement led to its rapid obsolescence by the early 1880s, as arc lamps with automatic feed mechanisms proved more practical for sustained operation.

Flame Arc Lamp

The flame arc lamp emerged in the late 1880s as a refinement of the carbon arc lamp, incorporating chemical additives to enhance color rendering and produce a whiter light suitable for indoor applications. Invented by Hugo Bremer in 1889, this design involved modifying carbon electrodes by coring or coating them with rare-earth salts, such as cerium fluoride or samarium fluoride, which vaporized during operation to create a luminous "flame" effect. These additives shifted the light output from the bluish tint of plain carbon arcs toward a more balanced spectrum, addressing the limitations of earlier open-arc designs for environments requiring natural illumination. In operation, the flame arc lamp functioned similarly to the basic carbon arc, with an electric discharge between two electrodes, but the salts stabilized the arc by forming a plasma envelope as they vaporized, necessitating higher currents of 20-40 amperes to maintain the flame. This process generated a yellow-white light with a color temperature of approximately 3000 K, providing a warmer glow than untreated arcs. The electrodes, typically consisting of a high-grade carbon shell around a core of 50% fluoride salts, required careful regulation to prevent instability, and the lamp was often used in series circuits for public or commercial settings. The spectrum of the flame arc lamp featured a broader visible range compared to plain carbon arcs, with reduced excess in the blue wavelengths and prominent emissions from metal ions, such as cerium lines in the green-yellow region, resulting in improved color rendering. Efficacy reached about 3-5 lumens per watt, owing to the incandescent vapor contributions rather than solely electrode incandescence. These characteristics made flame arc lamps advantageous for indoor use, such as in theaters and shops, where their less harsh, more flattering light enhanced visibility and aesthetics without the starkness of unenhanced arcs. Despite these benefits, flame arc lamps suffered from drawbacks including accelerated electrode erosion due to the reactive salts, which shortened operational life to 100-200 hours, and higher costs from the rare materials involved in electrode production. The need for ventilation to manage fumes from the vaporizing compounds further complicated installation, limiting widespread adoption beyond specialized applications.

Applications

Public Lighting and Lighthouses

Arc lamps revolutionized public street lighting in the late 19th century by enabling series-connected systems that powered multiple lamps from a single dynamo, typically supporting 20 to 100 units depending on the generator's capacity. In Cleveland, Ohio, inventor Charles F. Brush demonstrated this technology in 1879 by installing 12 arc lamps, each rated at 2,000 candlepower, on ornate 150-foot towers around Public Square, creating the first permanent outdoor electric lighting installation and replacing dimmer gas lamps with far brighter, more reliable illumination for safer nighttime navigation. This setup not only enhanced visibility but also supported municipal goals of brighter streets, which were believed to deter crime and extend business hours into the evening. Similar advancements followed in European and American cities, where arc lamps mounted on towers up to 150 feet high provided broad coverage over urban areas. Paris installed its first arc streetlights in 1878 using Yablochkov candles, marking the earliest large-scale electric public lighting in Europe and allowing for safer, more vibrant nightlife that boosted commercial activity after dark. In New York City, the Brush Electric Light & Power Company erected 12 arc lamps along Broadway from 14th to 26th Street in December 1880, powered by a central station, which similarly transformed downtown illumination and facilitated longer operating hours for shops and theaters while contributing to perceptions of reduced urban crime through improved visibility. These installations highlighted arc lamps' superiority over gas lighting, offering intensities of 2,000 to 4,000 candlepower per unit for widespread dispersal without the flicker or soot associated with older methods. In maritime navigation, arc lamps proved invaluable for lighthouses due to their high intensity when paired with Fresnel lenses, which focused the light into powerful beams visible for miles at sea. The South Foreland Upper Lighthouse in England pioneered this application in 1858, becoming the world's first electrically powered lighthouse with a Holmes arc lamp driven by a steam-powered dynamo, producing a beam that dramatically outperformed traditional oil lamps in reliability and reach. Following this success, U.S. lighthouses began adopting arc lighting in the 1870s, integrating carbon arc sources within Fresnel optics to enable safer coastal passages during the peak era of sail and early steam shipping. Despite their advantages, arc lamp systems for public and lighthouse use faced significant operational hurdles, including the need for weatherproof enclosures to protect against rain and wind, as the open-arc design was vulnerable to environmental exposure. Maintenance was labor-intensive, requiring nightly trimming and replacement of carbon electrodes to sustain the arc, often performed by dedicated crews under challenging conditions. Power generation relied on steam engines coupled to dynamos, which demanded constant fuel and oversight, further complicating deployment in remote lighthouse settings or expansive street networks. Arc lamps reached their zenith in public lighting during the 1880s and 1890s, with over 130,000 units illuminating U.S. streets by 1890 and approximately 158,000 still in service by 1900, reflecting global adoption estimated in the hundreds of thousands before incandescent bulbs began supplanting them for their lower maintenance and adaptability. This period marked a transformative shift in urban and maritime safety, though the technology's decline underscored the trade-offs between brilliance and practicality.

Projection Systems

Arc lamps were instrumental in the evolution of early projection systems, particularly as light sources for magic lanterns and motion picture projectors starting in the late 19th century. Their intense white light, generated by the electric arc between carbon electrodes, provided the necessary brightness—often exceeding 5,000 lumens—to project clear images onto large screens in darkened theaters, marking a significant advancement over oil or gas lamps. This capability facilitated the public screening of films from the 1890s onward, with systems like those developed by the Lumière brothers relying on carbon arcs to illuminate 35mm film strips for audiences. The high luminous output enabled early cinema to transition from peep-show devices, such as Edison's Kinetoscope, to shared viewing experiences in vaudeville houses and dedicated cinemas. In military and maritime signaling, carbon arc lamps powered searchlights that became prominent in the 1890s, offering focused beams for long-distance visibility. Equipped with parabolic mirrors to concentrate the light, these searchlights achieved high intensities enabling detection ranges of several miles under clear conditions—critical for naval engagements and coastal patrols during conflicts like the Spanish-American War. The U.S. Navy, for instance, integrated carbon arc searchlights on warships by the mid-1890s, enhancing nighttime operations by illuminating targets or signaling allies without revealing positions prematurely. This application leveraged the arc's directional efficiency, distinct from its broader use in area lighting. Theater lighting also benefited from arc lamps, which evolved from limelights in the late 19th century to become the preferred source for spotlights, enabling precise control and artistic effects. By the 1880s, carbon arcs replaced calcium-based limelights in many European and American stages, producing a steady, high-temperature beam (around 3,500 K) that could be filtered through colored gels for mood lighting or dynamic fades. This shift allowed directors to spotlight actors selectively, as seen in productions at venues like the Lyceum Theatre in London, where arcs facilitated innovative scene transitions and atmospheric depth previously limited by gas illumination. The lamps' portability and intensity supported the rise of modern stagecraft, influencing designs by pioneers like David Belasco. Technically, arc lamp projection systems employed optical components such as condenser lenses to gather and focus divergent rays from the arc onto the film gate, paired with reflectors—often silvered glass or polished metal—to collimate the output into a parallel beam for uniform screen coverage. Enclosed variants, common in high-power cinema setups, incorporated water-cooling jackets around the electrodes and housing to dissipate the intense heat (up to 4,000°C at the arc crater), preventing carbon vaporization and ensuring stable operation for hours. Operators manually adjusted electrode spacing via motorized feeds to maintain the arc gap, typically 2-6 mm, while ventilation systems managed fumes from electrode erosion. These setups demanded skilled projectionists, as inconsistent arcs could cause flicker or film damage. Carbon arc projection endured into the mid-20th century, powering film theaters through the 1920s and 1930s before gradual replacement by xenon short-arc lamps in the 1950s, which offered longer life (over 1,000 hours versus arcs' 50-100 hours) and reduced maintenance without sacrificing intensity. Xenon systems provided a more consistent spectrum closer to daylight, eliminating the need for constant electrode trimming and improving safety by containing the arc in a quartz envelope. Despite this shift, carbon arcs' legacy in high-brightness projection influenced later formats, underscoring their role in establishing visual media standards.

Contemporary Uses

Enclosed arc lamps, particularly short-arc xenon variants, continue to serve specialized high-brightness applications where broad-spectrum illumination is essential. These DC-operated lamps, typically ranging from 1000 to 20,000 watts with a color temperature around 5000 K, are employed in IMAX and cinema projectors to deliver intense, daylight-like light for large-scale projections. They also power endoscopy equipment for precise internal imaging, providing stable UV-to-visible output in medical procedures. Additionally, short-arc xenon lamps function as solar simulators in research settings, replicating sunlight spectra for photovoltaic testing and material analysis. High-pressure mercury and metal halide arc lamps find use in industrial and niche lighting, despite environmental concerns. Mercury short-arc lamps support UV curing processes in printing and coatings, lithography for semiconductor fabrication, and aquariums for photosynthetic growth in marine environments, achieving efficacies up to 100 lm/W. Metal halide variants enhance color rendering in aquariums, promoting coral health with balanced spectral output. However, these lamps contain hazardous mercury, prompting strict regulations and recycling initiatives worldwide. In scientific contexts, enclosed arc lamps provide reliable broadband sources for advanced instrumentation. Xenon arcs pump dye lasers in continuous-wave operations, offering stable excitation for tunable outputs in research. They illuminate microscopy and spectroscopy setups, valued for their high radiance and minimal flicker across UV to near-IR wavelengths. These applications leverage xenon's high color rendering index exceeding 90, ensuring accurate spectral representation. Despite advantages like lifespans over 1000 hours with proper cooling, enclosed arc lamps are increasingly phased out post-2010s in favor of LEDs for energy efficiency and mercury-free operation. Niche high-brightness needs persist, supported by hybrid LED-arc systems in UV curing and projection to balance intensity and sustainability as of 2025. Regulations, such as Canada's prohibitions on mercury-containing lamps starting 2026, accelerate recycling programs and transitions.

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